Genetics and Homosexuality: Are People Born Gay?
The Biological Basis for Sexual Orientationby Rich Deem

Introduction

Born Gay?

There is a common belief among liberals that people are born either gay
or straight. Conservatives tend to believe that sexual orientation is
actually sexual preference, which is chosen by the individual. This page
represents a review of the scientific literature on the basis for homosexual orientation.

Rich Deem

Are people born gay or straight? Much of the current media sources assume
the question is a solved scientific problem with all the evidence pointing
toward a biological (probably genetic) basis for a homosexual orientation.
Contrary to this perception, the question has been poorly studied (or studied
poorly), although
there is some evidence on both sides of question. In addition, many of the
initial studies, which were highly touted by the media as "proof" for a
biological basis for homosexuality, have been contradicted by later, more
thorough studies. This evidence falls into
four basic categories:

Brain structure

Possible hormonal influences

Concordance of homosexuality in twins

Concordance of genetic markers in siblings

Real genetic studies (GWAS)

Why does it matter?

Until a few years ago, sexual orientation used to be called sexual
preference. Obviously, the two terms denote significant differences in the the
manner by which sexuality develops. A preference is something that is
chosen, whereas orientation is merely something that defines us. The
differences are potentially important regarding how the law applies to those
who are gay. If homosexuality is not chosen, but actually is a
biologically-determined characteristic over which we have no choice, then laws
should not treat gays and straights differently, since homosexuality would be
equivalent to one's race, over which we have no control.

Sexual orientation—brain studies

Since sexual attraction begins in the brain, researchers first examined the
question of sexual orientation by comparing the anatomy of brains from males and
females. These studies showed that male and female brains showed sexual
dimorphism in the pre-optic area of the hypothalamus, where males demonstrated a
greater than two-fold difference in cell number and size compared to females.1
A second study found that two of four Interstitial Nuclei of the Anterior
Hypothalamus (INAH) were at least twice as large in males as females.2
Since the INAH was involved in sexual dimorphism, it was hypothesized by Simon
LeVay that there might be differences in this region in heterosexual vs.
homosexual men. Postmortem examination of the brains of AIDS patients vs.
control male subjects (presumed to be heterosexual) showed that the presumably
heterosexual men exhibited INAH3 that were twice the size of both females and
presumably homosexual men who had died of AIDS.3
The study has been criticized for its uncertainty of sexual orientation of the
subjects, and potential complications caused by the AIDS virus (which does
infect the human brain), and also by lowered testosterone levels found in AIDS patients. A popularized Newsweek cover story, "Is This
Child Gay?"4 characterized LeVay as a "champion for
the genetic side," even though the study involved no genetic data at all.

A subsequent study by Byne, et al. examined the question of
INAH3 size on the basis of sex, sexual orientation, and HIV status.5
The study found large differences in INAH3 volume on the basis of sex
(with the male INAH3 being larger than the female INAH3). However, the
volume of IHAH3 was decreased in male heterosexual men who had
contracted AIDS (0.108 mm3
compared with 0.123 mm3 in male controls). There was no
statistically significant difference between IHAH3 sizes of male heterosexuals vs.
male homosexuals who had contracted AIDS (0.108 mm3 and 0.096
mm3, respectively). The study also found that there were no
differences in the number of neurons in the INAH3 based upon sexual
orientation, although researchers found significant differences between males
and females, as in other studies.5 It was obvious from this
study that LeVay's study was fatally flawed due to the AIDS
complication, and that there were no differences in the INAH3 based upon
sexual orientation.

The role of the hypothalamus in sexual orientation was further
studied by Swaab, et al. Other researchers had hypothesized
that differentiation of the hypothalamus occurred before birth. However,
Swaab's study showed that the sexually dimorphic nucleus (SDN) of more
than 100 subjects decreased in volume and cell number in the females
only 2-4 years postnatal. This finding complicated the findings of the brain
studies, since not only chemical and hormonal factors, but also social factors,
might have influenced this process.6

A study by Allen and Gorski examined the anterior commissure of the
brain, finding that females and homosexual males exhibited a
larger size than heterosexual males.7
However, later studies using larger sample sizes found no such
differences.8

Complicating the issue of brain differences between homosexuals and
heterosexuals is the problem that sexual experiences themselves can
affect brain structure.9 So, the question
will always be whether homosexual practice changes the brain or whether
the brain results in homosexual practice.

Hormonal influences

Since sexual differentiation occurs within the womb, as a result of
hormonal influences, it has been hypothesized that homosexuality may
result from a differential hormone balance in the wombs of those who
eventually exhibit a homosexual orientation. Since hormonal levels
within the womb are not available, proxies for hormonal influences have
been used to examine the question of how hormonal influences might
impact sexual orientation. These proxies include differences in skeletal
size and shape, including the ratio of the long bones of the arms and
legs relative to arm span or stature and the hand bones of adults (the
ratio of the length of the various phalanges).

Studies have shown that ratios of digit length are predictors of
several hormones, including testosterone, luteinizing hormone and
estrogen.10 In women, the index finger (2D,
second digit) is almost the same length as the fourth digit (4D).
However, in men, the index finger is usually shorter than the fourth. It
has been shown that this greater 2D:4D ratio in females is established
in two-year-olds. It has been hypothesized that the sex difference in
the 2D:4D ratio reflects the prenatal influence of androgen on males.
A study by Williams, et al. showed that the 2D:4D ratio of homosexual men was not significantly different from
that of heterosexual men for either hand.11 However, homosexual women
displayed significantly smaller 2D:4D ratios compared with heterosexual
women (see figure to right). It has been
hypothesized that women exposed to more androgens in the womb tend to
express a homosexual orientation. However, since these hormone levels
were never measured, one is left with the proxy of finger lengths as a
substitute. Studies have found that the more older
brothers a boy has, the more likely he is to develop a homosexual
orientation.12 This study also found that homosexual men had
a greater than expected proportion of brothers among their older
siblings (229 brothers: 163 sisters) compared with the general population
(106 males: 100 females). Males who had two or more older brothers were
found to have lower 2D:4D ratios,11
suggesting that they had experienced increased androgens in the womb.
Why increased androgens would predispose both males and females to be
homosexual was not explained in the study.

Another study examined the length of long bones in the arms, legs and
hands. Both homosexual males and heterosexual females had less long bone
growth in the arms, legs and hands, than heterosexual males or
homosexual females.13 Accordingly, the
researchers hypothesized that male homosexuals had less androgen
exposure during development than male heterosexuals, while female
homosexuals had greater steroid exposure during development than their
heterosexual counterparts. Of course, with regard to male homosexuality,
this study directly contradicted the presumed results of the Williams
study above, which "showed" that males with multiple older brothers (who
tended to be homosexual) experienced increased androgen exposure.

A study of one homosexual vs. two heterosexual male triplets
found that the homosexual triplets scored more on the female side of the
Masculinity-Femininity scale of the Minnesota Multiphasic Personality
Inventory,14 suggesting a possible
hormonal influence (decreased androgens) involved in male homosexual orientation.

All of the studies reporting possible hormonal influence on
homosexuality suffer from the lack of any real evidence that hormones
actually play any role in sexual orientation. The fact that contradictory
studies report increased11,15
vs. decreased13-14
androgens as a basis for homosexuality doesn't provoke confidence that
the proxies are really true. Obviously, a study that documented real hormone levels, as opposed to
proxies, would probably provide more definitive data.

Studies involving a rare hormonal imbalance, congenital adrenal
hyperplasia (CAH), caused by defective 21-hydroxylase enzyme, suggest that
hormonal abnormalities can influence sexual orientation. CAH results in
increased production of male hormones during development. In males,
increased androgens has little effect. However, female fetuses that develop
in this environment develop ambiguous external genitalia, which complicates
subsequent development. In utero treatment with dexamethasone reduces the
androgen imbalance, resulting in an individual who is genetically and
phenotypically female. However, dexamethasone treatment also results in
reduced homosexual orientation among treated females,16
suggesting that some homosexuality may result from hormonal influences
during development. Homosexual rights groups have suggested that
dexamethasone treatment not be given, because it reduces homosexual
orientation in females affected by CAH.

Twin studies

The observation that familial factors influence the prevalence of
homosexuality led to a the initiation of number of twin studies, which are a proxy for the
presence of possible genetic factors. Most of these early studies
suffered from methodological flaws. Kallmann sampled subjects from
correctional and psychiatric institutions—not exactly representative
"normal" populations.17 Bailey
et al.
published a number of studies in the early 1990's, examining familial
factors involved in both male and female homosexuality. These studies
suffered from the manner in which subjects were recruited, since the
investigators advertised in openly gay publications, resulting in skewed populations.18 Later studies by the
same group did not suffer from this selection bias, and found the
heritability of homosexuality in Australia was up to 50 and 60% in females but only
30% in males.19

A study by Kendler et al. in 2000 examined 1,588 twins
selected by a random survey of 50,000 households in the United States.20
The study found 3% of the population consisted of non-heterosexuals
(homosexuals and bisexuals) and a genetic concordance rate of 32%,
somewhat lower than than found in the Australian studies. The study lost
statistical significance when twins were broken down into male and
female pairs, because of the low rate (3%) of non-heterosexuals in the
general U.S. population.

A Finnish twin study reported the "potential for homosexual response,"
not just overt homosexual behavior, as
having a genetic component.21

On a twist on homosexual twin studies, an Australian research group examined the question
of whether homophobia was the result of nature or nurture.22 Surprisingly,
both familial/environmental and genetic factors seemed to play a role as
to whether or not a person was homophobic. Even more surprising, a
separate research group in the U.S. confirmed these results (also adding
that attitudes towards abortion were also partly genetic).23 Now,
even homophobes can claim that
they were born that way!

Twin studies suffer from the problem of trying to distinguish between
environmental and genetic factors, since twins tend to live within the
same family unit. A study examining the effect of birth order on
homosexual preference concluded, "The lack of relationship between the
strength of the effect and degree of homosexual feelings in the men and
women suggests the influence of birth order on homosexual feelings was
not due to a biological, but a social process in the subjects studied."12
So, although the twin studies suggest a possible genetic component for
homosexual orientation, the results are certainly not definitive.

Genetic studies—the "gay gene"

An examination of family pedigrees revealed that gay men had more
homosexual male relatives through maternal than through paternal lineages,
suggesting a linkage to the X chromosome. Dean Hamer24 found such an
association at region Xq28. If male sexual orientation was influenced by a gene
on Xq28, then
gay brothers should share more than 50% of their alleles at this region,
whereas their heterosexual brothers should share less than 50% of their
alleles. In the absence of such an association, then both types of brothers
should display 50% allele sharing. An analysis of 40 pairs of gay brothers and found that they shared 82% of their
alleles in the Xq28 region, which was much greater than the 50% allele sharing
that would be expected by chance.25 However, a follow-up study
by the same research group, using 32 pairs of gay
brothers and found only 67% allele sharing, which was much closer to the 50%
expected by chance.26 Attempts by Rice et
al. to repeat the Hamer study resulted in only 46% allele sharing,
insignificantly different from chance, contradicting the Hamer results.27
At the same time, an unpublished study by Alan Sanders (University of
Chicago) corroborated the Rice results.28
Ultimately, no gene or gene product from the Xq28 region was ever
identified that affected sexual orientation. When Jonathan Marks (an
evolutionary biologist) asked Hamer what percentage of homosexuality he
thought his results explained, his answer was that he thought it explained
5% of male homosexuality. Marks' response was, "There is no science
other than behavioral genetics in which you can leave 97.5% of a phenomenon
unexplained and get headlines."29

Abusive childhood experiences

A study of 13,000 New Zealand adults (age 16+) examined sexual
orientation as a function of childhood history.30
The study found a 3-fold higher prevalence of childhood abuse for those who
subsequently engaged in same sex sexual activity. However, childhood abuse
was not a majorfactor in homosexuality, since only 15% of
homosexuals had experienced abuse as children (compared with 5% among
heterosexuals).30 So, it would appear
from this population that only a small percentage of homosexuality (~10%) might
be explained by early childhood abusive experiences.

Sexual preference or orientation?

If homosexual orientation were completely genetic, one would expect
that it would not change over the course of one's life. For females,
sexual preference does seem to change over time. A 5-year study of
lesbians found that over a quarter of these women relinquished their
lesbian/bisexual identities during this period: half reclaimed
heterosexual identities and half gave up all identity labels.31 In a
survey of young minority women (16-23 years of age), half of the
participants changed their sexual identities more than once during the
two-year survey period.32 In another study of subjects who were recruited from
organizations that serve lesbian/gay/bisexual youths (ages 14 to 21
years) in New York City, the percentage that changed from a
lesbian/gay/bisexual orientation to a heterosexual orientation was 5%
over the period of just 12 months (the length of the survey).33 Other studies
have confirmed that sexual orientation is not fixed in all individuals, but
can change over time, especially in women.34
A recent example of an orientation change occurred with The
Advocate's "Person of the Year" for 2005. Kerry Pacer was the
youngest gay advocate, chosen for her initiation of a "gay-straight
alliance" at White County High School in Cleveland, Georgia. However,
four years later, she is raising her one year old daughter, along with
the baby's father.35 Another former
lesbian, British comedienne Jackie Clune, spent 12 years in lesbian
relationships before marrying a man and producing 4 children.36
Michael Glatze came out at age 20 and went on to be a leader in the
homosexual rights movement. At age 30, he came out in the opposite
direction, saying, "In my experience, "coming out" from under the
influence of the homosexual mindset was the most liberating, beautiful
and astonishing thing I've ever experienced in my entire life."37
A 2011 study of Christian gays who wanted to change their
sexual orientation found that 23% of the subjects reported a successful
"conversion" to heterosexual orientation and functioning, while an
additional 30% reported stable behavioral chastity with substantive
dis-identification with homosexual orientation.38
However, 20% of the subjects reported giving up on the process and fully
embraced a gay identity, while another 27% fell in between the two
extremes.38
Obviously, for at
least some individuals, being gay or straight is something they can
choose.

The question of nature vs. nurture can also be seen by examining children
of homosexual vs. heterosexual parents. If homosexuality were purely
biological, one would expect that parenting would not influence it. Paul
Cameron published a study in 2006 that claimed that the children of
homosexual parents expressed a homosexual orientation much more frequently
than the general population.39 Although claims
of bias were made against the study, another study by Walter Schuum in 2010
confirmed Cameron's results by statistically examining the results of 10
other studies that addressed the question.40
In total, 262 children raised by homosexual parents were included in the
analysis. The results showed that 16-57% of such children adopted a
homosexual lifestyle. The results were even more striking in daughters of
lesbian mothers, 33% to 57% of whom became lesbians themselves. Since
homosexuals makeup only ~5% of the population, it is clear that parenting
does influence sexual orientation.

It always amazes me when people say that they were born gay. Looking back
on my own experience, I would never say that I was "born straight." I really
didn't have any interest in females until about the seventh grade. Before
that time, they weren't really interesting, since they weren't interested in
sports or riding bikes or anything else I liked to do.

Homosexuality and Darwinism

I am not a huge fan of Neo Darwinian evolution. Nevertheless, there is some
clear evidence that natural selection (and sexual selection) does act upon
populations and has acted on our own species to produce racial differences.41
Natural selection postulates that those genetic mutations that favor
survival and reproduction will be selected, whereas those that compromise
survival and reproduction will be eliminated. Obviously, a gene or series of
genes that produce non-reproducing individuals (i.e., those who express pure
homosexual behavior) will be rapidly eliminated from any population. So, it
would be expected that any "gay gene" would be efficiently removed from a
population. However, it is possible that a gene favoring male
homosexuality could "hide" within the human genome if it were located on the
X-chromosome, where it could be carried by reproducing females, and not be
subject to negative selection by non-reproducing males. In order to survive, the gene(s) would be expected to be
associated with higher reproductive capacity in women who carry it
(compensating for the generation of non-reproducing males). I can't imagine
a genetic scenario in which female homosexuality would ever persist within a
population.

Real genetic studies?

Within the last decade, genetic analysis of heritable traits has
taken a huge step forward with the advent of DNA microarray technology.
Using this technology, it is possible to scan large lengths of the human
genome (even an entire genome wide scan—GWAS) for numerous
individuals, at quite reasonable costs. This DNA microarray technology
has led to the discovery of genes that are associated with complex
diseases, such as Crohn's Disease, which is the topic of
my research. If
homosexuality truly has a genetic component, DNA microarray studies
would not only definitively prove the point, but would identify specific
gene(s) or loci that might be associated with those who express a
homosexual orientation. The first attempt to do genome wide scans on
homosexual males was done by Mustanski et al. in 2005.42
The results suggested possible linkage near microsatellite D7S798 on
chromosome 7q36. However, an attempt to repeat the finding (along with
~6000 well-defined SNPs spread comparatively evenly across the human
genome) failed to find any significant SNPs.43
However, a third study using Chinese subjects found a weak association
at the SHH rs9333613 polymorphism of 7q36.44
A more general study, examining mate choice among different populations,
found no genetic link, prompting the investigators to speculate that
such choices were "culturally driven."45
The largest genome wide scan was conducted by 23andMe. 7887 unrelated
men and 5570 unrelated women of European ancestry were analyzed by GWAS.
Although unpublished, the data was presented at the
American
Society of Human Genetics annual meeting in San Francisco, showing
that there were no loci associated with sexual orientation, including
Xq28 on the X chromosome.46
So, the preliminary studies on possible genetic causes of homosexual
orientation tends to rule out any dramatic genetic component to sexual
orientation.

Conclusion

Why are people gay? The question of how homosexual orientation originates has been the subject
of much press, with the general impression being promoted that homosexuality
is largely a matter of genes, rather than environmental factors. However, if
one examines the scientific literature, one finds that it's not quite as clear
as the news bytes would suggest. The early studies that reported differences
in the brains of homosexuals were complicated by HIV infection and were not
substantiated by larger, better controlled studies. Numerous studies reported
that possible hormonal differences affected homosexual orientation. However,
these studies were often directly contradictory, and never actually
measured any hormone levels, but just used proxies for hormonal influences,
without direct evidence that the proxies were actually indicative of true
hormone levels or imbalances. Twin studies showed that there likely are
genetic influences for homosexuality, although similar studies have shown some
genetic influences for homophobia and even opposition to abortion. Early
childhood abuse has been associated with homosexuality, but, at most, only
explains about 10% of those who express a homosexual orientation. The fact
that sexual orientation is not constant for many individuals, but can change
over time suggests that at least part of sexual orientation is actually sexual
preference. Attempts to find a "gay gene" have never identified any gene or
gene product that is actually associated with homosexual orientation, with
studies failing to confirm early suggestions of linkage of homosexuality to
region Xq28 on the X chromosome. The question of genetic influences on sexual
orientation has been recently examined using DNA microarray technology,
although, the results have largely failed to pinpoint any specific genes as a
factor in sexual orientation.

Dean Hamer gained even more notoriety by publishing a
book entitled The God Gene: How Faith Is Hardwired into Our Genes,
which a
Scientific American Review of The God Gene said should have been titled, "A Gene That
Accounts for Less than One Percent of the Variance Found in Scores on
Psychological Questionnaires Designed to Measure a Factor Called
Self-Transcendence, Which Can Signify Everything from Belonging to the Green
Party to Believing in ESP, According to One Unpublished, Unreplicated
Study."